This invention relates to earthquake-resistant anchoring systems.
Earthquakes comprise horizontal and vertical ground vibrations. During an earthquake, structural bodies such as buildings, nuclear power plants, oil and chemical storage tanks, and the like structures, which are connected to the earth by conventional foundations are subjected to forced vibrations which are imposed onto these structural bodies by the movement of their respective foundations. The inertia of the structural body tends to resist the earthquake-induced movements of the foundation. As a result, a lateral shearing force (base shear) is applied to the structural body at its foundation. The magnitude of this base shear is a major factor in earthquake damage and is the principal concern of the structural designer. Inasmuch as the forces to which a structural body is subjected during an earthquake are directly proportional to the mass of the structural body, these forces can be minimized to some extent through the utilization of lightweight materials of construction and by designing structures of relatively low total weight; however, the structural designer is limited in his approaches because of the types and cost of currently commercially available materials of construction.
In the case of a nuclear power facility, potential earthquake damage constitutes a special safety problem because of the possibility that fission products may be released. Accordingly, a relatively higher safety factor is required to avoid the possibility of exposing the population to excessive radiation.
The usual design approach is to use available materials and to size all structural members so as to withstand an earthquake of predetermined level of severity. It has to be kept in mind, however, that earthquakes do not have a "windward" side or a "leeward" side, thus each and every structural body or object has to be secured against earthquake-generated forces. While in the case of buildings the vertical vibrations generated during an earthquake are relatively small and may be disregarded for design purposes, in a nuclear power plant the effects of vertical as well as horizontal vibrations have to be combined directly and linearly with sources of stress from dead load, live load, thermal effects, pressures, and other applicable operating conditions and loadings when determining the design maxima for all sources of stress that may be encountered. The net results are very high design and construction costs.
The present invention obviates or at least minimizes many of the aforementioned difficulties and provides an earthquake-resistant anchoring system whereby the stresses to which a structural body is subjected during an earthquake can be attenuated. Moreover, the present invention provides a system which can be constructed at a relatively low cost. Also, the present anchoring system provides an elastic connection between the structural body and the earth which does not transmit vibrations to the structural body but which effectively immobilizes the structural body.